U.S. patent application number 14/501816 was filed with the patent office on 2015-04-16 for imaging optical system having bending optical element.
This patent application is currently assigned to HOYA CORPORATION. The applicant listed for this patent is HOYA CORPORATION. Invention is credited to Hiroshi NOMURA, Eijiroh TADA.
Application Number | 20150103417 14/501816 |
Document ID | / |
Family ID | 52809437 |
Filed Date | 2015-04-16 |
United States Patent
Application |
20150103417 |
Kind Code |
A1 |
NOMURA; Hiroshi ; et
al. |
April 16, 2015 |
IMAGING OPTICAL SYSTEM HAVING BENDING OPTICAL ELEMENT
Abstract
An imaging optical system includes a bending optical element
which bends an object-emanating light bundle, a post-bending lens
system on a post-bending optical axis defined by the bending
optical element, and an image sensor. An effective optical surface
of a large-diameter lens element, having a greatest axial light
bundle effective radius, is formed into a non-circular shape by
making a length of the effective optical surface from the
post-bending optical axis toward a side opposite from the object
side smaller than the axial light bundle effective radius, with
reference to the axial light bundle effective radius lying on a
plane which extends orthogonal to a plane including both the
post-bending optical axis and a pre-bending optical axis of the
imaging optical system and includes the post-bending optical
axis.
Inventors: |
NOMURA; Hiroshi; (Saitama,
JP) ; TADA; Eijiroh; (Saitama, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HOYA CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
HOYA CORPORATION
Tokyo
JP
|
Family ID: |
52809437 |
Appl. No.: |
14/501816 |
Filed: |
September 30, 2014 |
Current U.S.
Class: |
359/733 |
Current CPC
Class: |
G02B 13/005 20130101;
G02B 13/04 20130101; G02B 13/0065 20130101 |
Class at
Publication: |
359/733 |
International
Class: |
G02B 13/00 20060101
G02B013/00; G02B 13/04 20060101 G02B013/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2013 |
JP |
2013-214525 |
Claims
1. An imaging optical system, comprising: a bending optical element
which bends a light bundle emanating from an object; a post-bending
lens system arranged on a post-bending optical axis defined by an
optical axis of said imaging optical system being bent by said
bending optical element; and an image sensor, on which an image of
said object is formed via said post-bending lens system; wherein an
effective optical surface of a large-diameter lens element, having
an axial light bundle effective radius that is greatest among those
of all lens elements of said post-bending lens system, is
configured into a non-circular shape by a length of said effective
optical surface from said post-bending optical axis toward a side
opposite from the object side being smaller than said axial light
bundle effective radius, with reference to said axial light bundle
effective radius lying on a plane which includes said post-bending
optical axis and extends orthogonal to a plane including both said
post-bending optical axis and a pre-bending optical axis of said
imaging optical system.
2. The imaging optical system according to claim 1, wherein said
axial light bundle effective radius that is greatest among those of
all lens elements of said post-bending lens system is determined
with reference to said axial light bundle effective radius lying on
a plane which includes said post-bending optical axis and extends
orthogonal to a plane including both said post-bending optical axis
and a pre-bending optical axis of said imaging optical system.
3. The imaging optical system according to claim 1, wherein said
non-circular shape of said effective optical surface satisfies the
following conditions: RU>RL, and (RU+RL)<2RH, wherein, when
viewed in a direction along said post-bending optical axis, where a
U-direction designates a direction parallel to a pre-bending
optical axis and travelling toward the object side, an L-direction
designates a direction opposite to said U-direction, and an
H-direction designates a direction that is orthogonal to a plane on
which both said pre-bending optical axis and said post-bending
optical axis lie, RU designates a distance between said
post-bending optical axis and an outermost point of said effective
optical surface of said large-diameter lens element in said
U-direction within the axial light bundle effective radius, RL
designates a distance between said post-bending optical axis and an
outermost point of said effective optical surface of said
large-diameter lens element in said L-direction within said axial
light bundle effective radius, and RH designates a distance between
said post-bending optical axis and an outermost point of said
effective optical surface of said large-diameter lens element in
said H-direction within said axial light bundle effective
radius.
4. The imaging optical system according to claim 3, wherein said
non-circular shape of said effective optical surface further
satisfies the following condition: 0.5<{(RU+RL)/2RH}<0.9.
5. The imaging optical system according to claim 3, wherein said
effective optical surface of said large-diameter lens element is
circular in shape except in said L-direction, and wherein, in said
L-direction, said effective optical surface of said large-diameter
lens element comprises a straight side, which is parallel to said
post-bending optical axis and orthogonal to a plane on which both
said post-bending optical axis and said pre-bending optical axis
lie, to define a D-cut shape.
6. The imaging optical system according to claim 1, further
comprising a first lens element on said object side of said bending
optical element.
7. The imaging optical system according to claim 1, wherein said
imaging optical system is a retrofocus type in which a negative
lens group and a positive lens group are arranged, as a whole, in
that order from said object side, and wherein said large-diameter
lens element is included in said positive lens group.
8. The imaging optical system according to claim 1, wherein said
large-diameter lens element is at a position at which an entrance
pupil of said imaging optical system is similar in shape to said
effective optical surface of said large-diameter lens element.
9. An imaging optical system, comprising: a bending optical element
which bends a light bundle emanating from an object; a post-bending
lens system on a post-bending optical axis defined by an optical
axis of said imaging optical system being bent by said bending
optical element; and an image sensor, on which an image of said
object is formed via said post-bending lens system, wherein an
outer shape of an effective optical surface of a large-diameter
lens element, an axial light bundle effective radius of which is
greatest among all lens elements of said post-bending lens system,
is formed to be non-circular as viewed in a direction of said
post-bending optical axis, and wherein a shape of an entrance pupil
of said imaging optical system satisfies the following conditions:
RU'>RL', and (RU'+RL')<2RH', wherein, when the entrance pupil
is viewed in a direction along a pre-bending optical axis, where an
L'-direction designates a direction parallel to said post-bending
optical axis which travels toward the image sensor side, a
U'-direction designates a direction opposite to said L'-direction,
and an H'-direction designates a direction orthogonal to a plane on
which both a pre-bending optical axis and said post-bending optical
axis lie, RU' designates a distance between said pre-bending
optical axis and an outermost point of the entrance pupil in said
U'-direction, RL' designates a distance between said pre-bending
optical axis and an outermost point of the entrance pupil in said
L'-direction, and RH' designates a distance between said
pre-bending optical axis and an outermost point of the entrance
pupil in said H'-direction.
10. The imaging optical system according to claim 9, wherein said
shape of said entrance pupil further satisfies the following
condition: 0.5<{(RU'+RL')/2RH'}<0.9.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an imaging optical system
having a bending optical element.
[0003] 2. Description of the Related Art
[0004] In recent years, mobile electronic devices which are
designed mainly for taking still/moving photographic images, such
as digital cameras (still-video cameras) and digital camcorders
(motion-video cameras), and other mobile electronic devices which
are designed to be capable of taking such photographic images as a
subsidiary function, such as mobile phones equipped with a camera
and tablet computers, etc., equipped with a camera, have become
widespread, and there has been a demand to miniaturize the imaging
units incorporated in these types of mobile electronic devices. In
order to miniaturize an imaging unit, it is known to configure an
optical system of an imaging unit as a bending optical system which
reflects (bends) a light bundle using a reflection surface of a
reflecting element (bending optical element) such as a prism or a
mirror. An imaging optical system having at least one bending
optical element (hereinafter also referred to as a "bending optical
system") is advantageous in achieving a reduction in thickness of
the imaging unit, especially in the travelling direction of the
incident light emanating from an object to be photographed.
[0005] On the other hand, in an imaging optical system, which can
be reduced in thickness, the imaging optical system has also been
required to have a small F-number. Designing the imaging optical
system to have a small F-number usually causes the lens diameter to
increase, thus being incompatible with the demand for slimming. In
the case of an imaging optical system which includes a reflection
surface (bending optical element) in a lens group that is provided
closest to the object side, since a lens group that allows a light
bundle having a large axial light-bundle effective diameter (axial
light bundle diameter) to pass through lies on a post-bending
optical axis (i.e., an optical axis optically behind the reflection
surface), if the imaging optical system is designed to have a small
F-number, the aforementioned lens group (lying on the post-bending
optical axis) also increases in size, which inhibits a reduction in
thickness of the imaging unit.
[0006] A non-circular lens element, which is formed as a circular
lens element with an outer edge section thereof partly cut off to
miniaturize the lens element (or reduce the diameter of the lens
element), is known in the related art (disclosed in Japanese
Unexamined Patent Publication Nos. 2006-267391, 2010-24376 and
2013-105049). However, the non-circular lens elements disclosed in
the above-mentioned disclosures of the related art are each
designed based on the technical idea of removing portions of the
lens element through which only light rays that reach the outer
side of the imaging surface pass due to the imaging surface (image
pickup device/image sensor) of an imaging optical system, which
includes such a non-circular lens, being rectangular (non-circular)
in shape. Accordingly, in such non-circular lens elements of the
related art, sections of the lens element through which only
off-axis light rays reaching the outer side of the imaging surface
pass and are positioned outside the axial light bundle effective
radius are merely cut off. The axial light bundle effective radius
is defined by the length of a perpendicular line which connects the
optical axis with a point of intersection between an optical
surface of the lens element and a light ray which passes through
the outermost peripheral edge of the lens element among a group of
light rays (light bundle) emerging from an object point on the
optical axis and reaching an image point on the optical axis
through the optical system. Hence, the reduction in diameter
(miniaturization) of the lens (lens group) is not sufficient, and
accordingly, the bending optical system cannot be sufficiently
reduced in thickness even any such non-circular lens elements of
the related art are adopted.
SUMMARY OF THE INVENTION
[0007] Based on the above described technical background, the
present invention provides an imaging optical system having one or
more bending optical elements, wherein the imaging optical system
makes it possible to strike a balance between a reduction in
F-number and a further reduction in thickness of the imaging
optical system.
[0008] The present invention has been devised to achieve a further
reduction in thickness of the imaging optical system by cutting off
a specific portion of an effective optical surface (a surface
through which light rays involved in image formation pass) of a
lens element within the axial light bundle effective radius (i.e.,
cutting out a portion of the light rays that are involved in image
formation) with reference to the axial light bundle effective
radius.
[0009] The present invention concentrates on the shape of an
effective optical surface of a large-diameter lens element having
the greatest axial light-bundle effective radius among the lens
elements that lie on a post-bending optical axis of an imaging
optical system; accordingly, the present invention has been devised
from the viewpoint that it is possible to reduce the thickness of
the imaging optical system while suppressing deterioration of the
optical performance thereof via image processing if the
large-diameter lens element is formed into an irregular shape
(non-circular shape) by making the length of the effective optical
surface from the post-bending optical axis toward the side opposite
from the object side smaller than the axial light bundle effective
radius, with reference to the axial light bundle effective radius
lying on a plane which includes the post-bending optical axis and
extends orthogonal to a plane including both the post-bending
optical axis and the pre-bending optical axis of the imaging
optical system. If an effective optical surface of a large-diameter
lens element is formed into a non-circular shape, the light bundle
incident on an imaging surface becomes asymmetrical, so that a
difference in resolving power theoretically occurs between each
direction (vertical and horizontal directions). However, no
practical problems occur if aberrations are sufficiently corrected,
and the problem of peripheral light quantity being asymmetrical can
also be corrected by image processing. Additionally, in imaging
optical systems, a negative or positive lens element can usually be
arranged in front of a bending optical element (on the object side
thereof), so that the length of a lens group in the post-bending
optical system from the optical axis thereof toward the object side
also does not have to be sacrificed.
[0010] According to an aspect of the present invention, an imaging
optical system is provided, including a bending optical element
which bends a light bundle emanating from an object; a post-bending
lens system arranged on a post-bending optical axis defined by an
optical axis of the imaging optical system being bent by the
bending optical element; and an image sensor, on which an image of
the object is formed via the post-bending lens system. An effective
optical surface of a large-diameter lens element, having an axial
light bundle effective radius that is greatest among those of all
lens elements of the post-bending lens system, is formed into a
non-circular shape by making a length of the effective optical
surface from the post-bending optical axis toward a side opposite
from the object side smaller than the axial light bundle effective
radius, with reference to the axial light bundle effective radius
lying on a plane which includes the post-bending optical axis and
extends orthogonal to a plane including both the post-bending
optical axis and a pre-bending optical axis of the imaging optical
system.
[0011] The axial light bundle effective radius that is greatest
among those of all lens elements of the post-bending lens system
can be determined with reference to the axial light bundle
effective radius lying on a plane which includes the post-bending
optical axis and extends orthogonal to a plane including both the
post-bending optical axis and a pre-bending optical axis of the
imaging optical system.
[0012] It is desirable for the non-circular shape of the effective
optical surface to satisfy the following conditions (1) and
(2):
RU>RL, and (1)
(RU+RL)<2RH, (2)
[0013] wherein, when viewed in a direction along the post-bending
optical axis, assuming a U-direction designates a direction
parallel to a pre-bending optical axis and travelling toward the
object side, an L-direction designates a direction opposite to the
U-direction, and an H-direction designates a direction that is
orthogonal to a plane on which both the pre-bending optical axis
and the post-bending optical axis lie,
[0014] RU designates a distance between the post-bending optical
axis and an outermost point of the effective optical surface of the
large-diameter lens element in the U-direction within the axial
light bundle effective radius,
[0015] RL designates a distance between the post-bending optical
axis and an outermost point of the effective optical surface of the
large-diameter lens element in the L-direction within the axial
light bundle effective radius, and
[0016] RH designates a distance between the post-bending optical
axis and an outermost point of the effective optical surface of the
large-diameter lens element in the H-direction within the axial
light bundle effective radius.
[0017] If the conditions (1) and (2) are not satisfied, it would be
difficult to obtain the effect of reducing the thickness of the
imaging optical system.
[0018] It is desirable for the non-circular shape of the effective
optical surface to further satisfy the following condition (3):
0.5<{(RU+RL)/2RH}<0.9. (3)
[0019] If the value "(RU+RL)/2RH" becomes less than or equal to
0.5, the difference in resolving power between each direction
(vertical and horizontal directions) becomes great, thereby causing
deterioration in image quality. If the value "(RU+RL)/2RH" becomes
greater than or equal to 0.9, the effect of reducing the thickness
becomes insufficient.
[0020] It is desirable for the effective optical surface of the
large-diameter lens element to be circular in shape except in the
L-direction (RU=RH), and, in the L-direction, for the effective
optical surface of the large-diameter lens element to include a
straight side, which is parallel to the post-bending optical axis
and orthogonal to a plane on which both the post-bending optical
axis and the pre-bending optical axis lie, to form a D-cut
shape.
[0021] It is desirable for the imaging optical system to include
one of a negative lens element and a positive lens element on the
object side of the bending optical element.
[0022] It is desirable for the imaging optical system to be a
retrofocus type in which a negative lens group and a positive lens
group are arranged, as a whole, in that order from the object side,
and for the large-diameter lens element to be included in the
positive lens group.
[0023] The irregular-shaped (non-circular) large-diameter lens
element in the imaging optical system which satisfies the
conditions (1) and (2) (and (3)) is arranged at a position at which
an entrance pupil of the imaging optical system is similar in shape
to the effective optical surface of the large-diameter lens
element. It is generally the case that an aperture stop installed
in an imaging optical system is arranged at a position where an
off-axis principal ray intersects the optical axis of the imaging
optical system, and that the area of the aperture stop changes the
quantity of light reaching an imaging surface; however, the shape
(a circle, a rectangle, a triangle, etc.) of the aperture stop as
viewed in the optical axis direction does not change the light
quantity distribution on the imaging surface. In contrast, the
irregular-shaped large-diameter lens element in the imaging optical
system according to the present invention is arranged in the
vicinity of an aperture stop at a position which defines the shape
of the entrance pupil of the imaging optical system (at a position
which influences the shape of the entrance pupil), and the shape of
an effective optical surface of the irregular-shaped large-diameter
lens element also influences the light quantity distribution on the
imaging surface.
[0024] In other words, the imaging optical system according to the
present invention achieves a reduction in thickness thereof is
achieved while a desired lens speed (F-number) is obtained (in
other words, the lens speed is not sacrificed) by setting the shape
of an effective optical surface of the irregular-shaped
large-diameter lens element so that, with reference to the area of
a circular-shaped entrance pupil for obtaining a desired lens speed
(F-number), the area of the irregular-shaped entrance pupil becomes
identical to the area of this circular-shaped entrance pupil.
Namely, an imaging optical system identical in lens speed to that
having a circular entrance pupil can be obtained by satisfying
"(the area of the entrance pupil formed by an axial light bundle
through a circular aperture)=(the area of the entrance pupil formed
by an axial light bundle through an irregular-shaped aperture)." In
this connection, in order to use an F-number definition in an
optical system having an irregular-shaped aperture, the F-number is
calculated from the diameter of the aforementioned reference
circular entrance pupil since the F-number is equal to the focal
length divided by the entrance pupil diameter (F-number=focal
length/entrance pupil diameter).
[0025] According to another aspect of the present invention an
imaging optical system is provided, including a bending optical
element which bends a light bundle emanating from an object; a
post-bending lens system arranged on a post-bending optical axis
defined by an optical axis of the imaging optical system being bent
by the bending optical element; and an image sensor, on which an
image of the object is formed via the post-bending lens system. An
outer shape of an effective optical surface of a large-diameter
lens element, an axial light bundle effective radius of which is
the greatest among all lens elements of the post-bending lens
system, is formed to be non-circular as viewed in a direction of
the post-bending optical axis, and a shape of an entrance pupil of
the imaging optical system satisfies the following conditions (1')
and (2'):
RU'>RL', and (1')
(RU'+RL')<2RH', (2')
[0026] wherein, when the entrance pupil is viewed in a direction
along a pre-bending optical axis, assuming an L'-direction
designates direction parallel to the post-bending optical axis and
travelling toward the image sensor side, a U'-direction designates
a direction opposite to the L'-direction, and an H'-direction
designates a direction orthogonal to a plane on which both the
pre-bending optical axis and the post-bending optical axis lie,
[0027] RU' designates a distance between the pre-bending optical
axis and an outermost point of the entrance pupil in the
U'-direction,
[0028] RL' designates a distance between the pre-bending optical
axis and an outermost point of the entrance pupil in the
L'-direction, and
[0029] RH' designates a distance between the pre-bending optical
axis and an outermost point of the entrance pupil in the
H'-direction.
[0030] It is desirable for the shape of the entrance pupil to
further satisfy the following condition (3'):
0.5<{(RU'+RL')/2RH'}<0.9. (3')
[0031] The present invention achieves both a reduction in F-number
and a further reduction in thickness of the imaging optical system
by providing a bending optical system which bends alight bundle
emanating from an object, a post-bending lens system which is
positioned on a post-bending optical axis that is defined by the
optical axis of the imaging optical system being bent by the
bending optical element, and an image sensor on which an image of
the object is formed via the post-bending lens system; and by the
outer shape of an effective optical surface of an irregular-shaped
large-diameter lens group, the axial light bundle effective radius
of which is the greatest among all the lens elements of the
post-bending lens system, being formed to be non-circular as viewed
in the direction of the post-bending optical axis.
[0032] The present disclosure relates to subject matter contained
in Japanese Patent Application No. 2013-214525 (filed on Oct. 15,
2013) which is expressly incorporated herein by reference in its
entirety.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The present invention will be described below in detail with
reference to the accompanying drawings in which:
[0034] FIG. 1 is a perspective external view of an embodiment of an
imaging unit to which the present invention is applied;
[0035] FIG. 2 is a perspective view of the imaging unit with the
housing removed, illustrating the internal structure of the imaging
unit;
[0036] FIG. 3 is a transverse sectional view of the imaging unit
taken along the longitudinal direction thereof;
[0037] FIG. 4 is a diagram showing the optical arrangement of the
optical elements of an imaging optical system, to which the present
invention has been applied;
[0038] FIG. 5 is a front elevational view, viewed in the direction
of the arrows V shown in FIG. 4, of an irregular-shaped
large-diameter lens element in the imaging optical system, showing
an example of the shape of an effective optical surface of the
irregular-shaped large-diameter lens element; and
[0039] FIG. 6 is a front elevational view, viewed in the direction
of the arrows VI shown in FIG. 4, of the entrance pupil of the
imaging optical system shown in FIG. 4, showing an example of the
shape of the entrance pupil.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0040] An embodiment of an imaging unit (imaging apparatus) 10
according to the present invention will be discussed below with
reference to FIGS. 1 through 6. In the following descriptions,
forward and rearward directions, leftward and rightward directions,
and upward and downward directions are determined with reference to
the directions of the double-headed arrows shown in FIGS. 1 through
6. The object side corresponds to the front side. As shown by the
outward appearance of the imaging unit 10 in FIG. 1, the imaging
unit 10 has a laterally elongated shape which is slim in the
forward/rearward direction and long in the leftward/rightward
direction.
[0041] As shown in FIGS. 2 and 3, the imaging unit 10 has an
imaging optical system which is provided with a first lens group
G1, a second lens group (post-bending optical system) G2, a third
lens group (post-bending optical system) G3 and a fourth lens group
(post-bending optical system) G4. The first lens group G1 is
provided with a first prism (bending optical element) L11, and the
imaging unit 10 is provided with a second prism L12 on the
right-hand side of the fourth lens group G4. The imaging optical
system of the imaging unit 10 is configured as a bending optical
system which reflects (bends) a light bundle at substantially right
angles at each of the first prism L11 and the second prism L12. As
shown in FIG. 3, the first lens group G1 is configured of a first
lens element L1, the first prism L11 and a second lens element L2.
The first lens element L1 is positioned in front of (on the object
side of) an incident surface L11-a of the first prism L11, while
the second lens element L2 is positioned on the right-hand side of
an exit surface L11-b of the first prism L11. Each of the second
lens group G2, the third lens group G3 and the fourth lens group G4
is a lens group which does not include a reflector element such as
a prism.
[0042] As shown in FIG. 3, an object-emanated light bundle along a
first optical axis (pre-bending optical axis) O1, which extends in
the rearward direction from the front of the imaging unit 10, and
is incident on the first lens element L1 enters the first prism L11
through the incident surface L11-a and is reflected by a reflection
surface L11-c of the first prism L11 toward a direction along a
second optical axis O2 (post-bending optical axis) (extending in
the rightward direction) to exit from the exit surface L11-b of the
first prism L11. Subsequently, the light bundle exiting from the
exit surface L11-b passes through the second lens element L2 of the
first lens group G1 and the second through fourth lens groups G2,
G3 and G4 (the second through fourth lens groups G2, G3 and G4
constituting a post-bending lens system), which lie on the second
optical axis O2, and enters into the second prism L12 through an
incident surface L12-a thereof. Subsequently, the light bundle
which is passed through the incident surface L12-a is reflected by
a reflection surface L12-c of the second prism L12 in a direction
along a third optical axis O3 (extending in the forward direction)
and is incident on the imaging surface of an image sensor IS to
form an object image thereon. The first optical axis O1 and the
third optical axis O3 are substantially parallel to each other and
lie, together with the second optical axis O2, on a common plane.
This common (imaginary) plane defines a first reference plane
(plane including both the pre-bending optical axis and the
post-bending optical axis) P1 (see FIG. 5) in which the first
optical axis O1, the second optical axis O2 and the third optical
axis O3 lie, and an imaginary plane which is orthogonal to the
first reference plane P1 and includes the second optical axis O2 is
represented by a second reference plane P2 (see FIG. 5). The
imaging unit 10 has a shape elongated in a direction along the
second optical axis O2, and the first lens element L1 is positioned
in the vicinity of an end (the left end) of the imaging unit 10
with respect to the lengthwise direction thereof.
[0043] As shown in FIGS. 1 through 3, the imaging unit 10 is
provided with a body (main) module 11 which holds the second lens
group G2, the third lens group G3, the fourth lens group G4, the
second prism L12 and the imaging sensor IS, and a first lens-group
unit 12 which holds the first lens group G1. The body module 11 is
provided with a box-shaped housing 13 which is elongated in the
leftward/rightward direction and is small in thickness (slim) in
the forward/rearward direction. The first lens-group unit 12 is
fixed to one end (the left end) of the housing 13, with respect to
the lengthwise direction thereof, and the fourth lens group G4, the
second prism L12 and the imaging sensor IS are fixedly held at the
other end (the right end) of the housing 13, with respect to the
lengthwise direction thereof. However, the configuration of the
imaging unit 10 is not limited to the embodiment shown in FIGS. 1
through 3; for example, the body module 11 and the first lens-group
unit 12 can alternatively be formed as a single unitary member.
[0044] As shown in FIG. 2, the second lens group G2 and the third
lens group G3 are held by a second lens group frame 20 and a third
lens group frame 21, respectively, which are supported to be
movable along the second optical axis O2 by a pair of rods 22 and
23 provided in the housing 13. The imaging unit 10 is provided with
a first motor M1 and a second motor M2 which are supported by the
housing 13. When the first motor M1 is driven to rotate a screw
shaft M1a thereof which projects from the body of the first motor
M1, this rotation is transmitted to the second lens group frame 20
to move the second lens group frame 20 along the pair of rods 22
and 23. When the second motor M2 is driven to rotate a screw shaft
M2a thereof which projects from the body of the second motor M2,
this rotation is transmitted to the third lens group frame 21 to
move the third lens group frame 21 along the pair of rods 22 and
23. The imaging optical system of the imaging unit 10 is a zoom
lens system (variable-focal length lens system), and a zooming
operation (power-varying operation) is performed by moving the
second lens group G2 and the third lens group G3 along the second
optical axis O2. In addition, a focusing operation is performed by
moving the third lens group G3 along the second optical axis
O2.
[0045] The imaging unit 10 is provided with an anti-shake (image
shake correction/image-stabilizing/shake reduction) system that
reduces image shake on an image plane which is caused by vibrations
such as hand shake. This anti-shake system drives the first lens
element L1 of the first lens group G1 in a plane orthogonal to the
first optical axis O1. This anti-shake system itself is unrelated
to the gist of the present invention, and therefore, the
description thereof is omitted from the following descriptions.
[0046] FIGS. 3 through 6 show an embodiment of the imaging optical
system of the imaging unit 10, according to the present invention,
which is directed to the outer shape (profile) of an effective
optical surface of the second lens group G2 (the outer shape of the
front effective optical surface of the second lens group G2 as
viewed from the left side with respect to FIGS. 3 and 4). This
embodiment is a result of the pursuance of the extreme reduction in
thickness of the imaging unit 10 (a reduction in the
forward/rearward direction thereof) by forming the second lens
group G2 into an irregular shape (non-circular). As shown in FIG.
4, the second lens group G2 is a large-diameter lens element
(non-circular large-diameter lens element), the axial light bundle
effective radius of which is the greatest among those of all lens
elements of all the lens groups (G2, G3 and G4) on the second
optical axis O2, so that the second lens group G2 becomes an
obstacle when attempting to reduce the thickness of the imaging
unit 10. The shape of the effective optical surface of the second
lens group G2 can be determined in a manner which will be discussed
hereinafter. Furthermore, although the second through fourth lens
groups G2 through G4 are shown in the drawings as single lens
elements, respectively, either or each of the second through fourth
lens groups G2 through G4 can be configured of a plurality of lens
elements.
[0047] First of all, the axial light bundle effective radius of a
lens element is a length defined as "the length of the
perpendicular line which connects the optical axis with a point of
intersection between an optical surface of the lens element and a
light ray which passes through the outermost peripheral edge of the
lens element among a group of light rays (light bundle) emerging
from an object point on the optical axis and reaching an image
point on the optical axis through the optical system". The
effective optical surface of the second lens group G2 (the surface
through which the light bundle reaching the image sensor IS passes)
is formed into an irregular shape (non-circular) by making the
length (RL) of the effective optical surface from the post-bending
optical axis toward the side opposite from the object side
(rearward direction) smaller than the axial light bundle effective
radius of the irregular shaped large-diameter lens element (the
second lens group G2), with reference to the axial light bundle
effective radius lying on the second reference plane P2.
Furthermore, the ratio of the length of the effective optical
surface of the irregular-shaped large-diameter lens element (the
second lens group G2) from the post-bending optical axis toward the
object side to the length of the effective optical surface of the
irregular-shaped large-diameter lens element from the post-bending
optical axis toward the side opposite the object side fall within a
fixed range.
[0048] Specifically, assuming that a U-direction designates the
direction parallel to the first optical axis O1 and travelling
toward the object side (the front side, i.e. the upper side with
respect to FIGS. 4 and 5) in the first reference plane P1, an
L-direction designates the direction opposite to the U-direction,
and an H-direction designates the direction of the second reference
plane P2, when the effective optical surface of the second lens
group G2 is viewed in a direction along the second optical axis O2
as shown in FIGS. 4 and 5, the outer shape of the effective optical
surface satisfies the following conditions (1) and (2):
RU>RL, and (1)
(RU+RL)<2RH, (2)
[0049] wherein RU designates the distance between the optical axis
(the second optical axis O2) and the outermost point of the
effective optical surface of the second lens group G2 in the
U-direction within the axial light bundle effective radius;
[0050] RL designates the distance between the optical axis (the
second optical axis O2) and the outermost point of the effective
optical surface of the second lens group G2 in the L-direction
within the axial light bundle effective radius; and
[0051] RH designates the distance between the optical axis (the
second optical axis O2) and the outermost point of the effective
optical surface of the second lens group G2 in the H-direction
within the axial light bundle effective radius (=the axial light
bundle effective radius in the second reference plane P2). As can
be understood from FIG. 5, strictly speaking, RH can be defined as
two distances, RH1 and RH2, which exist in diametrically opposite
directions. These two distances RH1 and RH2 are usually
substantially the same, however, in the case where one of the two
distances RH1 and RH2 is slightly shorter than the other, the
shorter of the two is all that is required to satisfy condition
(2); the same is true for condition (3) indicated below.
[0052] To satisfy the conditions (1) and (2) means to sacrifice
(partly obstruct) the light bundle involved in image formation
which passes through the effective optical surface of the second
lens group G2 within the axial light bundle effective radius. If
the shape of the effective optical surface of the second lens group
G2 is determined in this manner, the light bundle incident on the
imaging surface becomes asymmetrical in shape, thus advantageously
enabling a reduction in thickness of the imaging unit 10 even
though a difference (loss) in resolving power theoretically occurs
between each direction (vertical and horizontal directions). This
loss can be canceled out using an aberration correction technique,
and the problem of peripheral light quantity becoming asymmetrical
can also be corrected using image processing.
[0053] It is desirable for the shape of the effective optical
surface of the second lens group G2 to further satisfy the
following condition (3):
0.5<{(RU+RL)/2RH}<0.9. (3)
[0054] If the value "(RU+RL)/2RH" is equal to or less than 0.5, the
difference in resolving power between each direction (vertical and
horizontal directions) becomes great, thereby causing deterioration
in image quality. If the value "(RU+RL)/2RH" is equal to or greater
than 0.9, the effect of reducing the thickness becomes
insufficient.
[0055] In a desired embodiment, the effective optical surface of
the second lens group G2 is circular in shape, defined by the axial
light bundle effective radius (i.e., RU=RH, and RH1=RH2), except
for the side thereof in the L-direction having a D-cut shape;
whereas, the effective optical surface of the second lens group G2
in the L-direction defines a D-cut side (straight side) D1, which
is parallel to the second reference plane P2 as shown in FIG.
5.
[0056] An object of the present embodiment of the imaging unit 10
is to reduce, in particular, the distance RL in the L-direction on
the second lens group G2; the distance RU in the U-direction can be
greater than the distance RH in the H-direction (=the axial light
bundle effective radius) as required. Since the first lens element
L1 is arranged on the light-bundle incident side (the upper side
with respect to FIGS. 4 and 5) in the U-direction, the distance RU
in the U-direction being greater than the axial light bundle
effective radius does not adversely influence the slimming down
(reduction in thickness in the forward/rearward direction) of the
entire imaging unit 10, and likewise can be said in regard to the
radial directions between the U-direction and the H-direction.
[0057] Additionally, in the present embodiment of the imaging unit
10, the second lens group G2 is arranged at a position at which the
shape of the entrance pupil of the entire imaging optical system
becomes similar to the shape of the effective optical surface of
the second lens group G2. FIG. 6 shows the shape of the entrance
pupil EP of the imaging unit 10. The entrance pupil EP appears to
be similar in shape to the effective optical surface of the second
lens group G2. In FIG. 6, "P3" shown by a two-dot chain line
designates a plane which is orthogonal to the first reference plane
P1 and includes the first optical axis O1.
[0058] Accordingly, the entrance pupil EP is similar in shape to
the effective optical surface of the second lens group G2, and the
shape of the entrance pupil EP satisfies the following conditions
(1') and (2'); moreover, the shape of the entrance pupil EP further
satisfies the following condition (3'):
RU'>RL', (1')
(RU'+RL')<2RH', and (2')
0.5<[(RU'+RL')<2RH']<0.9. (3')
[0059] wherein, when the entrance pupil EP is viewed in a direction
along the pre-bending optical axis (the first optical axis O1),
assuming an L'-direction designates the direction parallel to the
post-bending optical axis and travelling toward the image sensor
side, a U'-direction designates the direction opposite to the
L'-direction, and an H'-direction designates the direction
orthogonal to a plane including both the pre-bending optical axis
and the post-bending optical axis (the first reference plane
P1),
[0060] wherein RU' designates a distance between the optical axis
(the first optical axis O1) and the outermost point of the entrance
pupil in the U'-direction;
[0061] RL' designates a distance between the optical axis (the
first optical axis O1) and the outermost point of the entrance
pupil in the L'-direction; and
[0062] RH' designates a distance between the optical axis (the
first optical axis O1) and the outermost point of the entrance
pupil in the H'-direction. As can be understood from FIG. 6,
strictly speaking, RH' can be defined as two distances, RH1' and
RH2', which exist in diametrically opposite directions. These two
distances RH1' and RH2' are usually substantially the same,
however, in the case where one of the two distances RH1' and RH2'
is slightly shorter than the other, the shorter of the two is all
that is required to satisfy condition (2'); the same is true for
condition (3') indicated below.
[0063] It is generally the case that an aperture stop installed in
an imaging optical system is arranged at a theoretical position at
which an off-axis principal ray intersects the optical axis of the
imaging optical system and that the area of the aperture stop
varies the quantity of light reaching the imaging surface and at
which the shape (a circle, a rectangle, a triangle, etc.) of the
aperture stop, as viewed in the optical axis direction, does not
change the light quantity distribution (peripheral light quantity)
on an imaging surface. In contrast, in the present embodiment of
the imaging unit 10, the second lens group G2 is not arranged at
the theoretical installation position of the aperture stop.
Therefore, although the shape of the effective optical surface of
the second lens group G2 causes the light quantity distribution on
the imaging surface to be asymmetrical, the problem of the light
quantity distribution on the imaging surface becoming asymmetrical
can be corrected by image processing, as mentioned above.
[0064] Although the optical system of the above illustrated
embodiment of the imaging unit 10 is provided with the second prism
L12, the present invention can also be applied to an imaging
optical system which includes no prisms corresponding to the second
prism L12. Additionally, although the second lens group G2, the
third lens group G3 and the fourth lens group G4 are provided on
the second optical axis O2, the present invention can also be
applied to a type of imaging optical system in which less than or
more than three lens groups are provided on an optical axis of the
imaging optical system which corresponds to the second optical axis
O2.
[0065] Additionally, in the first lens group G1, it is possible to
change the number of lens elements arranged in front of the
incident surface L11-a of the first prism L11 on the first optical
axis O1 and the number of lens elements arranged on the right-hand
side of the exit surface L11-b of the first prism L11 on the second
optical axis O2.
[0066] Although the imaging optical system of the above illustrated
embodiment of the imaging unit 10 is a zoom lens (variable power
optical system) which performs a zooming operation (power varying
operation) by moving the second lens group G2 and the third lens
group G3 along the second optical axis O2, the present invention is
also applicable to an imaging apparatus which incorporates an
imaging optical system having no power varying capability. For
instance, it is possible to modify the imaging unit 10 such that
the second lens group G2 and the third lens group G3 do not move
for a zooming operation and that the second lens group G2 or the
third lens group G3 moves solely for a focusing operation.
[0067] Although the incident surface L11-a of the first prism L11
in the above illustrated embodiment of the imaging unit 10 is in
the shape of a laterally elongated rectangle, the present invention
can also be applied to a type of imaging apparatus (imaging optical
system) having a first prism (which corresponds to the first prism
L11), the incident surface thereof having a different shape such as
a square or a trapezoid.
[0068] Obvious changes may be made in the specific embodiment of
the present invention described herein, such modifications being
within the spirit and scope of the invention claimed. It is
indicated that all matter contained herein is illustrative and does
not limit the scope of the present invention.
* * * * *